1. Field of the Disclosure
The disclosure generally relates to point-to-point transceivers, and more specifically to techniques to eliminate interference in point-to-point transceivers.
2. Related Art
Conventional microwave point-to-point transceivers, may typically utilize frequency domain duplexing (FDD) schemes with challenging isolation requirements between transmit and receive channels. Isolation requirements for transceivers exist to maximize receiver sensitivity to weak receive signals, from distant stations for example, or to maximize data throughput through a receive channel.
Spectrum management constraints may require transmit and receive channels to operate with minimal gap between one another. For example, when microwave transmit and receive channels are adjacent to one another, in some embodiments as close as 100 Mhz, the transmit signal and corresponding transmit noise may couple onto the receive antenna and the associated receive cabling and signal traces on PCBs, through various electromagnetic interference (EMI) paths, either radiated or conducted.
When the transmit signal itself, for example, couples into a sensitive receiver, the transmit signal, although adjacent to the receive channel, may saturate the receiver. In a similar way, corresponding transmit noise components, such as transmit signal harmonics, or intermodulation products, may directly overlap the receive channel, likewise saturating, or significantly reducing the sensitivity of the receiver to the intended receive signal. This effect is exacerbated, in some exemplary microwave point-to-point systems, where transmission signal power may exceed 30 dBm at a transmit antenna, while a corresponding receive channel may include signals that are received as low as −100 dBm at a receive antenna.
In other embodiments, the transmit and receive antennas may be collocated on a tower, or share a common antenna element, providing additional radiated and conducted paths for transmit signals and noise components to enter the receiver. Furthermore, in many cases, the degree or nature of transmit coupling into the receiver may be unpredictable at the time a microwave point-to-point system is designed. Coupling mitigation techniques as discussed above, that may not detect and adapt to changing or initially unpredictable conditions, such as temperature variations, atmospheric changes, or corrosion of transmit and receive antenna elements or feed lines, may not be sufficient to meet system requirements either at initial installation, or over the operational life of the system.
To minimize this unintended coupling, in many cases, RF circuit paths in receivers may include shielding assemblies to isolate receive PCB signal traces and cabling from radiated transmit signals or transmit noise coupling onto the signal traces. Likewise, PCB signal traces and cabling feeding sensitive receivers typically may include common-mode filters or physical isolation to prevent transmit signals and corresponding noise components from coupling into a sensitive receiver. These isolation techniques however may not reduce transmit coupling enough to meet the isolation requirements necessary for proper operation of the receiver, for example, to receive distant transmit station signals, or operate high bandwidth waveforms, particularly sensitive to noise in the receive channel. Thus, in some embodiments, in order to meet stringent isolation requirements between transmit and receive channels, microwave point-to-point systems may include specifically designed diplexers to share a common antenna between a receiver and transmitter.
Such diplexers may typically include, for example, a low pass filter coupled to a receive path, and a high pass filter coupled to a transmit path, both coupled to a common antenna. The low and high pass filters may provide isolation between the respective receive and transmit channels by designing the cutoff frequency of the respective filters in a way that attenuates interfering signals coupled from the opposite signal path. In many cases, the diplexer provides the required isolation between receive and transmit paths, however at a significant cost in component cost, size, and configurability.
Diplexers may be physically large due to the number and size of the inductors and capacitors necessary to provide the isolation and steep cutoff required by closely adjacent channels. Due to their large size, diplexers may be unable to take advantage of current surface mount packaging and installation techniques, instead requiring manual installation and tuning. Furthermore, point-to-point systems microwave systems may be unable to configure diplexers once they are installed due to the nature of their manufacture and isolation structures contained therein. Thus, diplexers may drive significant cost and design constraints on microwave point-to-point systems, and other communications systems.
Embodiments of the disclosure will now be described with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the reference number.